Link connected question

Hi,

I know this is probably an amazingly dumb question, but what *exactly* does
it mean when an Ethernet link is connected? Does that mean it can sense the
media, and it does some kind of test to make sure it can send and hear
uncorrupted frames on the segment?

It sounds like a really silly question, but I can't seem to find a definitive
answer to it anywhere, everything just says "if the link light is on, you're
connected"... ok, but what *exactly* does that mean? I ask more out of
curiosity than any practical reason :)

Thanks!

.